Membrane electrode and fuel cell using the same

ABSTRACT

A membrane electrode includes a first electrode, a second electrode, and a proton exchange membrane sandwiched between the first electrode and the second electrode. The first electrode includes a first gas diffusion layer and a first catalyst layer. The second electrode includes a second gas diffusion layer and a second catalyst layer. The first catalyst layer or the second catalyst layer includes a carbon nanotube-metal particle composite including carbon nanotubes, polymer layer, and metal particles. The polymer layer is coated on a surface of the carbon nanotubes and defines a plurality of pores uniformly distributed; the metal particles are located in the pores. A fuel cell including the membrane electrode is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201210231368.X, filed on Jul. 5, 2012, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. This application is related tocommon-assigned application entitled, “CARBON NANOTUBE-METAL PARTICLECOMPOSITE AND CATALYST COMPRISING THE SAME” filed ______ (Atty. DocketNo. US46418), entitled, “METHOD FOR MAKING CARBON NANOTUBE-METALPARTICLE COMPOSITE” filed ______ (Atty. Docket No. US45284).

BACKGROUND

1. Technical Field

The present disclosure relates to a membrane electrode and a fuel cellusing the membrane electrode.

2. Description of Related Art

A fuel cell is an electrochemical power device. The fuel cell canconvert a chemical energy of a fuel and an oxidant gas into anelectrical energy through an electrochemical reaction. The fuel cellsare usually classified as alkaline fuel cells, solid oxide fuel cells,and proton exchange membrane fuel cells.

The proton exchange membrane fuel cells commonly include a membraneelectrode, a current collecting plate, and a flow guide plate. Themembrane electrode includes a proton exchange membrane, a gas diffusionlayer and a catalyst layer. The catalyst layer commonly includes acatalyst, a catalyst carrier, a proton conductor, and adhesive. Ingeneral, the catalyst is nano-scale precious metal particles. Theprecious metal particles are easily aggregated and have an unevendiameter distribution. Thus, a utilization rate of the catalyst is low,and a work efficiency of the fuel cell is decreased.

What is needed, therefore, is to provide a membrane electrode having ahigh utilization rate of the catalyst, and a fuel cell using the same.

BRIEF DESCRIPTION OF THE DRAWING

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, the emphasis instead being placed upon clearlyillustrating the principles of the present embodiments.

FIG. 1 is a structural schematic view of one embodiment of a carbonnanotube-metal particle composite.

FIG. 2 is a distribution schematic view of a polymer and metal particlesin the carbon nanotube-metal particle composite of FIG. 1.

FIG. 3 is a structural schematic view of one embodiment of a membraneelectrode.

FIG. 4 is a structural schematic view of a fuel cell using the membraneelectrode of FIG. 3.

FIG. 5 is a scanning transmission electron microscope image of thecarbon nanotube-metal particle composite of FIG. 1.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone.

One embodiment of a method for making a carbon nanotube-metal particlecomposite includes the following steps:

S1, providing carbon nanotubes, polymer monomers, a first solutioncontaining metal ions, and a second solution containing carboxylic acidradical ions;

S2, mixing the carbon nanotubes and the polymer monomers in a solvent toform a first mixture, wherein the polymer monomers are adsorbed on asurface of the carbon nanotubes;

S3, forming a second mixture by mixing the first mixture, the firstsolution, and the second solution, wherein the polymer monomers, thefirst solution, and the second solution react with each other to form acoordination complex mixture containing the metal ions, the coordinationcomplex mixture is adsorbed on the surface of the carbon nanotubes; and

S4, adding a reducing agent into the second mixture to reduce the metalions of the coordination complex mixture to metal particles andsimultaneously polymerize the polymer monomers, thereby forming thecarbon nanotube-metal particle composite in situ.

In the step S1, the carbon nanotubes can be single-walled carbonnanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs),multi-walled carbon nanotubes (MWCNTs), or any combination thereof. Thecarbon nanotubes can be fabricated by a method of arc discharge,chemical vapor deposition (CVD), or laser evaporation. In oneembodiment, the MWCNTs fabricated by the CVD method are used. An innerdiameter of the MWCNTs can be in a range from about 10 nanometers toabout 50 nanometers. An outer diameter of the MWCNTs can be in a rangefrom about 30 nanometers to about 80 nanometers. A length of the MWCNTscan be in a range from about 50 microns to about 100 microns.

The polymer monomers and the carbon nanotubes are adsorbed with eachother. The polymer monomers can be aniline, pyrrole, thiophene, amide,propylene imine, or derivates thereof. The derivate, for example, can beacetanilide, methylpyrrole, ethylenedioxythiophene, oxamide orcaprolactam. The polymer monomers can be other substances capable ofhaving a polymerization to form a polymer coated on integral surface ofsingle or plural of carbon nanotubes. In one embodiment, the polymermonomers are aniline.

If the carbon nanotube-metal particle composite is to be used as acatalyst, the metal ions in the first solution can be precious metalions or some metal ions having a good catalytic performance. Theprecious metal ions can be at least one of gold ions (Au³⁺), silver ions(Ag⁺), platinum ions (Pt⁴⁺), rhodium ions (Rh³⁺), iridium ions (Ir⁴⁺).The metal ions having a good catalytic performance can be at least oneof copper ions (Cu²⁺), ferrous ions (Fe²⁺), cobalt ions (Co²⁺), andnickel ions (Ni²⁺). Accordingly, the first solution can be a salt or anacid solution containing the metal ions. The first solution can bechloroauric acid (HAuCl₄), gold chloride (AuCl₃), silver nitrate(AgNO₃), chloroplatinic acid (H₂PtCl₆), ruthenium chloride (RuCl₃),chlororhodic acid (H₃RhCl₆), palladium chloride (PdCl₂), hexachloroosmicacid (H₂OsCl₆), hexachloroiridic acid (H₂IrCl₆), copper sulfate (CuSO₄),ferrous chloride (FeCl₂), or any combination thereof. In one embodiment,the first solution is H₂PtCl₆ water solution. In addition, the carbonnanotube-metal particle composite can be used in other fields, thus, themetal ions are not limited to the above mentioned species.

The second solution can have a complex action with the metal ions andthe polymer monomers. Thus, the metal ions and the polymer monomers canbe uniformly dispersed in the second solution. The second solution canbe a carboxylic acid solution or a carboxylic salt solution. In oneembodiment, the carboxylic acid or the carboxylic salt in the secondsolution includes at least two carboxylic acid groups (—COO—). Thecarboxylic salt can be sodium citrate (Na₃C₆H₅O₇) or potassium citrate(K₃C₆H₅O₇). The carboxylic acid can be citric acid (C₆H₈O₇), oxalic acid(C₂H₂O₄), malonic acid (C₃H₄O₄), butane diacid (C₄H₆O₄), adipic acid(C₆H₁₀O₄), terephthalic acid (C₈H₈O₄), glutaric acid (C₅H₈O₄), or anycombination thereof. In one embodiment, the second solution is Na₃C₆H₅O₇water solution.

In the step S2, the polymer monomers are dissolved in the solvent andadsorbed on the surface of the carbon nanotubes. The solvent can beethanol, diethyl ether, water, or any combination thereof. To uniformlydisperse the carbon nanotubes in the solvent, the solvent can be amixing liquid of two substances. In one embodiment, the solvent is amixture of water and ethanol, and a volume ratio of the water to theethanol is about 1:1. A mass ratio of the carbon nanotubes to thepolymer monomers can be in a range from about 1:1 to about 1:7. In oneembodiment, the mass ratio of the carbon nanotubes to the polymermonomers is 1:1.

The step S2 can further include a step of agitating the first mixture.After agitating the first mixture, the polymer monomers can be uniformlyadsorbed on the surface of the carbon nanotubes, and the carbonnanotubes can be uniformly dispersed in the solvent. In one embodiment,the first mixture is ultrasonically dispersed for about 3 hours to about8 hours.

The step S2 can further include a step of functionalizing the carbonnanotubes by using hydrophilic groups before mixing the carbon nanotubesand the polymer monomers.

The hydrophilic groups can be carboxyl groups, hydroxide groups or amidegroups. In one embodiment, at least two carboxyl groups are provided bya carboxyl acid. The carboxyl acid can be C₆H₈O₇, C₂H₂O₄, C₃H₄O₄,C₄H₆O₄, C₆H₁₀O₄, C₈H₈O₄, C₅H₈O₄, or combinations thereof. One or some ofthe carboxyl groups are used to surface functionalize the carbonnanotubes, thereby increasing a dispersion ability of the carbonnanotubes in water. The spare carboxyl groups that do not react with thecarbon nanotubes are electrostatically attracted to the polymermonomers. Thus, an improved polymer coating of the carbon nanotubes canbe obtained. The polymer monomers are adsorbed on the surface of thecarbon nanotubes by the carboxyl groups.

In the step S3, a complex reaction occurs between the metal ions and thecarboxylic acid radical ions, and a complex reaction occurs between thepolymer monomers and the carboxylic acid radical ions, thereby formingthe coordination complex mixture adsorbing on the surface of the carbonnanotubes. A molar ratio of the carboxyl acid salt or the carboxyl acidin the second solution to the metal ions of the first solution can be ina range from about 1:1 to about 5:1. In one embodiment, the molar ratioof the Na₃C₆H₅O₇ in the second solution to Pt ions of the H₂PtCl₆ in thefirst solution is about 1:1. In addition, a molar ratio of the carboxylacid salt or the carboxyl acid in the second solution to the polymermonomers can be in a range from about 1:1 to about 1:6. In oneembodiment, the molar ratio of the carboxyl acid salt or the carboxylacid in the second solution to the polymer monomers is about 1:2.

In the step S3, the first mixture, the first solution and the secondsolution can be simultaneously or successively added into a reactor toform the second mixture.

In one embodiment, the step S3 further includes the sub-steps of:

S31, mixing the first solution and the second solution to form a mixingliquid; and

S32, adding the first mixture into the mixing liquid, thereby inducing areaction between the first mixture and the mixing liquid to form thecoordination complex mixture.

In the step S31, the carboxylic acid radical ions in the second solutionhave an excellent complexation. Thus, the metal ions existing in a formof complex ions can be stably dispersed in the mixing liquid. Thecarboxylic acid radical ions have a guide function to fix the reducedmetal particles on the carbon nanotubes.

Furthermore, the mixing liquid can be stirred or supersonicallydispersed. In one embodiment, the mixing liquid is supersonicallydispersed for about 8 hours to about 12 hours.

In the step S32, to introduce the complete reaction, the first mixturecan be slowly added into the mixing liquid. In addition, after addingthe first mixture into the mixing liquid, a potential difference betweenthe metal ions and the polymer monomers can be formed due to thecarboxylic acid radical ions, thereby partly reducing the metal ions andpartly oxidizing the polymer monomers. The carboxylic acid radical ionscan have a complex reaction with the polymer monomers. Besides, thecarboxylic acid radical ions can be also electrostatically attracted orchemically bonded with the polymer monomers. Thus, the carboxylic acidradical ions can be adsorbed on the surface of the carbon nanotubes. Inaddition, the carboxylic acid radical ions can also have a complexreaction with the metal ions to form a stable complex, whereby the metalions in the complex can be uniformly dispersed on the surface of thecarbon nanotubes.

In the step S3, a temperature of the complex reaction can be in a rangefrom about 4° C. to about 100° C. The temperature is related to a typeof the metal ions. In one embodiment, the temperature is about 15° C.

The step S3 further includes a step of adjusting a pH value of thesecond mixture, by which the coordination complex mixture can be stablydistributed in the solvent. In addition, the pH value of the secondmixture can be adjusted at the beginning of the complex reaction, andkept up until the reaction ends. The pH value of the second mixture canbe adjusted in a range from about 2 to about 5. In one embodiment, thepH value of the second mixture is adjusted to 3.

In the step S4, the polymer monomers are oxidized to form the polymeradsorbing on the surface of the carbon nanotubes under the action of thereducing agent. Meanwhile, the metal complex ions in the coordinationcomplex mixture are reduced to form the metal particles, thereby formingthe carbon nanotube-metal particle composite in situ. The reduction ofthe metal complex ions and the polymerization of the polymer monomersare simultaneous. Thus, the reduced metal particles are adsorbed betweenthe carbon nanotubes and the polymer. In addition, the carboxyl groupsin the second solution added in step S3 act as a bridge between themetal particles and the polymer. The carboxyl groups not onlyelectrostatically attract or chemically bond to the polymer but alsostrongly absorb the metal particle. The metal particles can be stablydispersed due to the spare carboxyl groups. In addition, the carboxylgroups can improve a phase separation between the polymer and the metalparticles. Thus, a plurality of pores are formed in the polymer, and themetal particles are located in the pores and adsorbed on the surface ofthe carbon nanotubes due to a guide function of the carboxyl groups. Adispersing uniformity of the metal particles in the carbonnanotube-metal particle composite can be improved due to the metalparticles being located in the uniformly distributed pores of thepolymer.

The reducing agent can reduce the metal ions to the metal particles. Thereducing agent can be at least one of sodium borohydride (NaBH₄),formaldehyde (CH₂O), hydrogen peroxide (H₂O₂), C₆H₈O₇, hydrogen (H₂),and ascorbic acid. In one embodiment, the reducing agent is the NaBH₄solution. A molar ratio of the reducing agent to the metal ions can bein a range from about 10:1 to about 60:1. In one embodiment, the molarratio of NaBH₄ to HAuCl₄ is about 50:1.

The carbon nanotube-metal particle composite is formed in situ due tothe carboxyl groups. The metal particles in a form of nanocluster have asmall diameter. The metal particles are adsorbed on the surface of thecarbon nanotubes. The nanocluster is a microscopic aggregation composedof a plurality of metal atoms combined with each other by physical orchemical force. In one embodiment, the metal particles are a nanoclustercomposed of less than or equal to 55 metal atoms.

In the step S4, the carbon nanotube-metal particle composite ispurified. Specifically, the carbon nanotube-metal particle composite canbe filtered and washed for many times. Furthermore, the carbonnanotube-metal particle composite can be dried.

The above method for making the carbon nanotube-metal particle compositeis simple. In the method, the metal particles are slowly reduced, thus,a shape of the metal particles can be easily controlled.

Referring to FIGS. 1 and 2, the carbon nanotube-metal particle composite100 includes carbon nanotubes 102, a polymer layer 104 coated on thesurface of the carbon nanotubes, and the metal particles 106.

The polymer layer 104 can be coated on the surface of the single carbonnanotube. A thickness of the polymer layer 104 can be in a range fromabout 1 nanometer to about 7 nanometers. The polymer layer 104 defines aplurality of pores 108. The plurality of pores 108 are uniformlydistributed. The metal particles 106 are located in the pores 108 andspaced from each other by the polymer of the polymer layer 104. Thus,the metal particles 106 are uniformly distributed on the surface of thecarbon nanotubes 102. In addition, the metal particles 106 in the pores108 can be directly adsorbed on the surface of the carbon nanotubes 102or partly inserted in the polymer layer 104, and exposed out from thesurface of the polymer layer 104. In one embodiment, only one metalparticle 106 is located in one pore 108. The metal particles 106 areadsorbed in the pores 108, thus, the metal particles 106 are uniformlydispersed in the carbon nanotube-metal particle composite 100. Adiameter of the metal particles 106 is in a range from about 1 nanometerto about 5 nanometers. In one embodiment, the diameter of the metalparticles 106 is in a range from about 1 nanometer to about 2nanometers. A mass percentage of the metal particles 106 to the carbonnanotube-metal particle composite 100 can be in a range from about 20%to about 70%. A material of the polymer layer 104 is a conductivepolymer, such as polyaniline (PANI), polypyrrole (PPY), polythiophene(PT), polyamide (PA), polypropyleneimine (PPI),poly(N-acetylaniline)(PNAANI), poly(N-methylpyrrole),poly(3,4-ethylendioxythiophene) (PEDT), polycaprolactam, or anycombination thereof.

A material of the metal particles 106 can be noble metal or catalystmetal. The noble metal can be gold (Au), silver (Ag), platinum (Pt),rhodium (Rh), iridium (Ir), or any combination thereof. The catalystmetal can be copper (Cu), iron (Fe), cobalt (Co), nickel (Ni), or anycombination thereof. The carbon nanotubes can be SWCNTs, DWCNTs, MWCNTs,or any combination thereof.

In one embodiment, the carbon nanotube-metal particle composite 100 isPt/PANI/MWCNT composite. The Pt particle is a nanocluster having adiameter of about 1 nanometer to about 2 nanometers.

In one embodiment, a catalyst is provided. The catalyst includes thecarbon nanotube-metal particle composite 100. In addition, the catalystcan further include other common catalyst material such as noble metalparticles. The catalyst can be used in different devices, such aselectrochemical reactors (e.g. membrane reactor using oil energy) orcells (e.g. fuel cells).

In the carbon nanotube-metal particle composite 100 of the catalyst, thecarboxyl groups can strongly adsorb the metal particles, therebyincreasing a loading quantity of the metal particles in the catalyst. Inaddition, the metal particles are uniformly dispersed in the carbonnanotube-metal particle composite 100, thereby increasing a utilizationrate of the catalyst. Thus, a catalyst property of the catalyst isimproved.

Referring to FIG. 3, one embodiment of a membrane electrode 200 includesa proton exchange membrane 202, a first electrode 204 and a secondelectrode 206. The proton exchange membrane 202 is sandwiched betweenthe first electrode 204 and the second electrode 206. The firstelectrode 204 includes a first gas diffusion layer 204 a and a firstcatalyst layer 204 b. The second electrode 206 includes a second gasdiffusion layer 206 a and a second catalyst layer 206 b. In the firstelectrode 204 and the second electrode 206, the catalyst layer isdisposed between the gas diffusion layer and the proton exchangemembrane 202.

At least one of the first catalyst layer 204 b and the second catalystlayer 206 b includes the carbon nanotube-metal particle composite 100.In one embodiment, both the first catalyst layer 204 b and the secondcatalyst layer 206 b include the carbon nanotube-metal particlecomposite 100. In another embodiment, both the first catalyst layer 204b and the second catalyst layer 206 b are composed of only the carbonnanotube-metal particle composite 100. The carbon nanotube-metalparticle composite 100 is uniformly distributed in the catalyst layer. Aquantity of the metal particles of the carbon nanotube-metal particlecomposite 100 in the catalyst layer can be in a range from about 0.3mg/cm² to about 2 mg/cm². In one embodiment, the quantity of the metalparticles is less than 0.4 mg/cm². The carbon nanotube-metal particlecomposite 100 can exist in a form of slurry. In the process forfabricating the membrane electrode 200, the carbon nanotube-metalparticle composite 100 in the form of the slurry is coated on thesurface of the proton exchange membrane 202 or the gas diffusion layer,and then the coated carbon nanotube-metal particle composite 100 isdried.

A material of the first gas diffusion layer 204 a can be the same as amaterial of the second gas diffusion layer 206 a. The material of thegas diffusion layer can be a porous material having a plurality ofpores, such as a carbon fiber paper or a carbon nanotube film includinga plurality of carbon nanotubes. In one embodiment, both the first gasdiffusion layer 204 a and the second gas diffusion layer 206 a are acarbon nanotube film. If the carbon nanotube-metal particle composite100 is coated on the surfaces of the first gas diffusion layer 204 a andthe second gas diffusion layer 206 a, the carbon nanotube-metal particlecomposite 100 can be distributed in a plurality of pores of the firstgas diffusion layer 204 a and the second gas diffusion layer 206 a.

A material of the proton exchange membrane 202 can be a proton exchangeresin containing sulfonic acid group. The proton exchange resin can beperfluorosulfonic acid resin or sulfonate polymer having a protonexchange function and excellent thermal stability. The sulfonate polymercan be sulfonated polyether sulphone resin, sulfonated polyphenylenesulfide resin, sulfonated polybenzimidazole resin, sulfonated phosphorusenrichment nitriles resin, sulfonated polyimide resin, sulfonatedpolystyrene-polyethylene copolymer resin, or any combination thereof.

Referring to FIG. 4, one embodiment of a fuel cell 300 includes themembrane electrode 200, a first guide plate 308 a, a second guide plate308 b, a first current collecting plate 310 a, a second currentcollecting plate 310 b, a first auxiliary element 312 a, and a secondauxiliary element 312 b.

The first guide plate 308 a is disposed on a surface of the firstelectrode 204 away from the membrane electrode 200. The second guideplate 308 b is disposed on a surface of the second electrode 206 awayfrom the membrane electrode 200. The first guide plate 308 a and thesecond guide plate 308 b can be used to transfer a fuel gas, an oxidantgas, or a reaction resultant (e.g. water). The first guide plate 308 aand the second guide plate 308 b are made of metal or conductivematerial (e.g. carbon). The first guide plate 308 a and the second guideplate 308 b define a plurality of guide grooves 314. The guide grooves314 are in contact with the gas diffusion layer and used to transfer thefuel gas, the oxidant gas, or the reaction resultant (e.g. water).

The first current collecting plate 310 a is disposed on a surface of thefirst guide plate 308 a away from the membrane electrode 200. The secondcurrent collecting plate 310 b is disposed on a surface of the secondguide plate 308 b away from the membrane electrode 200. The firstcurrent collecting plate 310 a and the second current collecting plate310 b are used to collect or transfer electrons. A material of the firstcurrent collecting plate 310 a and the second current collecting plate310 b can be a conductive material. The first current collecting plate310 a or the second current collecting plate 310 b is optional.

The first auxiliary element 312 a or the second auxiliary element 312 bcan include a blower (not shown), a pipe (not shown), and a valve (notshown). The blower is connected with the guide plate by the pipe. Theblower is used to provide the fuel gas or the oxidant gas. The firstauxiliary element 312 a and the second auxiliary element 312 b areoptional because the fuel gas or the oxidant gas can directly diffusetoward the fuel cell 300. In one embodiment, the fuel gas is hydrogen.The oxidant gas is oxygen or air containing oxygen.

In use, the fuel gas (H₂) is introduced into the second electrode 206through the second guide plate 308 b. The oxidant gas (e.g. oxygen gas,O₂) is introduced into the first electrode 204 through the first guideplate 308 a. The hydrogen gas is transferred to the second catalystlayer 206 b through the second gas diffuse layer 206 a. A reaction ofthe hydrogen gas can be executed under the catalysis of the catalyst. Anequation of the reaction can be: H₂→2H⁺+2e. The hydrogen ions producedby this reaction reach the first electrode 204 through the protonexchange membrane 202. The electrons are transferred to the externalcircuit.

On the other end of the fuel cell 300, the oxygen gas is introduced intothe first electrode 204. The electrons of the external circuit aretransferred to the first electrode 204. A reaction of the oxygen gas,the hydrogen ions, and the electrons can be executed under the catalysisof the catalyst. An equation of the reaction can be: ½O₂+2H⁺+2e→H₂O. Thewater produced by the above reaction can be expelled out through thefirst gas diffusion layer 204 a and the first guide plate 308 a.

In the above use process of the fuel cell 300, an electric potentialdifference is formed between the first electrode 204 and the secondelectrode 206. If a load 320 is connected with the external circuit, acurrent will be formed. The metal particles of the catalyst layer areuniformly dispersed and have a small diameter. Thus, the catalyst havingan excellent catalysis increases a reaction speed of the first electrode204 and the second electrode 206 and improves a current efficient and anoutput power. A power of the fuel cell 300 containing 1 gram metalparticles can be in a range from about 800 W/g to about 1500 W/g.

Example 1 Synthesis of the Pt/PANI/MWCNT Composite

MWCNTs and aniline are added into a first mixing liquid containing waterand ethanol to form the first mixture. A volume ratio of the water tothe ethanol is about 1:1. The first mixture is ultrasonically vibratedfor about 3 hours. A Na₃C₆H₅O₇ water solution and an H₂PtCl₆ watersolution are mixed to form a second mixing liquid. The second mixingliquid is ultrasonically vibrated for 8 hours. The first mixture is thenadded to the second mixing liquid to introduce a complex reaction underwater bath of about 15° C., thereby forming a coordination complexmixture. The mixture of the first mixture and the second mixing liquidis ultrasonically vibrated for about 1 hour. A mass ratio among theNa₃C₆H₅O₇, the aniline, and the MWCNTs is about 1:2:2. A NaBH₄ solutionis then added into the mixture of the first mixture and the secondmixing liquid to form a precipitate. A molar ratio of the Na₃C₆H₅O₇ tothe H₂PtCl₆ is about 50:1. The precipitate is then filtered and washed.The washed precipitate is dried for about 3 hours under about 60° C. toform the Pt/PANI/MWCNT composite. The Pt particles in a form of thenanocluster having a diameter of about 1 nanometer to about 2 nanometersare loaded on the MWCNTs or the PANI. A mass percentage of the Ptparticles to the Pt/PANI/MWCNT composite is about 30%. Referring to FIG.5, Pt particles are uniformly dispersed on the carbon nanotubes and thePANI.

Example 2 Synthesis of the Au/PANI/MWCNT Composite

The Au/PANI/MWCNT composite is synthesized by the same method as inExample 1, except that the HAuCl₄ solution replaces the H₂PtCl₆ solutionand a temperature of water bath is about 25° C. The Au particles in theAu/PANI/MWCNT composite are in a form of a nanocluster having a diameterof about 1 nanometer to about 3 nanometers. A mass percentage of the Auparticles to the Au/PANI/MWCNT composite is about 60%.

Example 3 Synthesis of the Fe/PANI/MWCNT Composite

The Fe/PANI/MWCNT composite is synthesized by the same method as inExample 1, except that the FeCl₂ solution replaces the H₂PtCl₆ solutionand a temperature of water bath is about 100° C. The Fe particles in theAu/PANI/MWCNT composite are in a form of a nanocluster having a diameterof about 2 nanometers to about 5 nanometers. A mass percentage of the Feparticles to the Fe/PANI/MWCNT composite is about 50%.

Example 4 A Preparation of the Catalyst Layer of the Fuel Cell

The catalyst layer is made of the Pt/PANI/MWCNT composite of theexample 1. During the process of making the catalyst layer, thePt/PANI/MWCNT composite and the perfluorosulfonic acid resin solutionare dispersed in an isopropyl alcohol water solution to form a catalystslurry. The proton exchange membrane is made of porous perfluorosulfonicacid film and immersed in the catalyst slurry. After immersing theporous perfluorosulfonic acid film, the porous perfluorosulfonic acidfilm is taken out from the catalyst slurry and dried to form thecatalyst layer on the porous perfluorosulfonic acid film. A carbonnanotube film and the porous perfluorosulfonic acid film with thecatalyst layer thereon are overlapped with each other and hot pressedtogether to form the membrane electrode. A monocell is assembled byusing the membrane electrode, a graphite guide plate, and apolytetrafluoroethylene seal ring. A power of the monocell containing 1gram metal particles is in a range from about 800 W/g to about 1000 W/g.

Depending on the embodiment, certain steps of methods described may beremoved, others may be added, and the sequence of steps may be altered.The description and the claims drawn to a method may include someindication in reference to certain steps. However, the indication usedis only to be viewed for identification purposes and not as a suggestionas to an order for the steps.

The above-described embodiments are intended to illustrate rather thanlimit the present disclosure. Variations may be made to the embodimentswithout departing from the spirit of the present disclosure as claimed.Elements associated with any of the above embodiments are envisioned tobe associated with any other embodiments. The above-describedembodiments illustrate the scope of the present disclosure but do notrestrict the scope of the present disclosure.

What is claimed is:
 1. A membrane electrode comprising: a firstelectrode comprising a first gas diffusion layer and a first catalystlayer; a second electrode comprising a second gas diffusion layer and asecond catalyst layer; and a proton exchange membrane located betweenthe first electrode and the second electrode; wherein at least one ofthe first catalyst layer and the second catalyst layer comprises acarbon nanotube-metal particle composite, the carbon nanotube-metalparticle composite comprises carbon nanotubes, polymer layer, and metalparticles; the polymer layer is coated on a surface of the carbonnanotubes and defines a plurality of uniformly distributed pores, themetal particles are located in the pores.
 2. The membrane electrode ofclaim 1, wherein the metal particles located in the pores are spacedfrom each other by a polymer of the polymer layer.
 3. The membraneelectrode of claim 1, wherein the metal particles located in the poresare uniformly distributed on the surface of the carbon nanotubes.
 4. Themembrane electrode of claim 1, wherein the metal particles are partlyinserted in the polymer layer and exposed out from the polymer layer. 5.The membrane electrode of claim 1, wherein a diameter of the metalparticles is in a range from about 1 nanometer to about 5 nanometers. 6.The membrane electrode of claim 1, wherein each of the metal particlesis a nanocluster having a diameter of about 1 nanometer to about 2nanometers.
 7. The membrane electrode of claim 6, wherein thenanocluster comprises metal atoms of less than or equal to
 55. 8. Themembrane electrode of claim 1, a mass percentage of the metal particlesto the carbon nanotube-metal particle composite is in a range from about20% to about 70%.
 9. The membrane electrode of claim 1, wherein apolymer of the polymer layer is selected from the group consisting ofpolyaniline, polypyrrole, polythiophene, polyamide, polypropyleneimine,poly(N-acetylaniline), poly(N-methylpyrrole),poly(3,4-ethylendioxythiophene), polycaprolactam or a combinationthereof.
 10. The membrane electrode of claim 1, wherein the surface ofthe carbon nanotubes is functionalized by hydrophilic groups.
 11. Themembrane electrode of claim 1, wherein the surface of the carbonnanotubes is functionalized by carboxyl groups, the polymer layer isadsorbed with the carbon nanotubes by the carboxyl groups.
 12. Themembrane electrode of claim 1, wherein only polymer layer is coated onsingle carbon nanotube.
 13. The membrane electrode of claim 1, wherein aquantity of the metal particles of the carbon nanotube-metal particlecomposite in the first catalyst layer or the second catalyst layer is ina range from about 0.3 mg/cm² to about 2 mg/cm².
 14. The membraneelectrode of claim 13, wherein the quantity of the metal particles ofthe carbon nanotube-metal particle composite in the first catalyst layeror the second catalyst layer is less than 0.4 mg/cm².
 15. The membraneelectrode of claim 1, wherein a material of the first catalyst layer orthe second catalyst layer is the carbon nanotube-metal particlecomposite.
 16. The membrane electrode of claim 1, wherein the carbonnanotube-metal particle composite is uniformly distributed on thesurface of the first gas diffusion layer or the second gas diffusionlayer, and sandwiched between the first gas diffusion layer and theproton exchange membrane, or the second gas diffusing layer and theproton exchange membrane.
 17. The membrane electrode of claim 1, whereinthe first gas diffusion layer or the second gas diffusion layer definesa plurality of pores, and the carbon nanotube-metal particle compositeis distributed in the pores.
 18. A membrane electrode comprising: afirst catalyst layer; a second catalyst layer; and a proton exchangemembrane sandwiched between the first catalyst layer and the secondcatalyst layer; wherein at least one of the first catalyst layer and thesecond catalyst layer comprises a carbon nanotube-metal particlecomposite, the carbon nanotube-metal particle composite comprises carbonnanotubes, polymer layer, and metal particles, the polymer layer iscoated on a surface of the carbon nanotubes and defines a plurality ofuniformly distributed pores, the metal particles are located in thepores.
 19. A fuel cell, comprising: a first guide plate; a second guideplate, and a membrane electrode sandwiched between the first guide plateand the second guide plate; wherein the membrane electrode comprises afirst catalyst layer; a second catalyst layer; and a proton exchangemembrane sandwiched between the first catalyst layer and the secondcatalyst layer; wherein at least one of the first catalyst layer and thesecond catalyst layer comprises a carbon nanotube-metal particlecomposite, the carbon nanotube-metal particle composite comprises carbonnanotubes, polymer layer, and metal particles, the polymer layer iscoated on a surface of the carbon nanotubes and defines a plurality ofuniformly distributed pores, the metal particles are located in thepores.
 20. The fuel cell of claim 19, wherein a power of the fuel cellcontaining the metal particles of 1 gram is in a range from about 800W/g to about 1000 W/g.